1. Field of the Invention
The present invention relates to a video coding and decoding method for an image communication system. Particularly, the present invention relates to a video coding and decoding method for setting a reference compression ratio for prevention of degradation of picture quality during video reconstruction, compressing a present image with change of the compression ratio in accordance with a transmission rate, comparing the compression ratio of the present compressed image with the reference compression ratio, controlling a compensation value of a video signal obtained before the compression to lower complexity of the video signal if the compression ratio of the present compressed image exceeds the reference compression ratio, and applying the compression ratio to the compensation value controlled image in coding video signals in an image communication system, thereby minimizing the degradation of transmitting image quality and increasing transmission efficiency.
2. Description of Related Art
FIG. 1 schematically illustrates a configuration of an image communication system according to a related art.
The image communication system allows the image and voice of a caller to be simultaneously transmitted through communication lines including cables, such as PSTN (Public Switched Telephone Network), dedicated lines and optical cable, and/or wireless networks.
Such image communication system comprises: a charge coupled device (CCD) 1 for converting images incoming through a lens 1a for taking a photograph into electronic video signals; a CCD driver 2 for driving the CCD 1 and processing the electronic video signals received from the CCD 1 to provide outputs; a digital signal processor 3 for digital signal processing the video signals output from the CCD driver 2 to output the signals through a monitor 4 and, simultaneously, transforming the video signals into a specified format (CCIR656 format) of luminance signals (Y) and color signals (Cb, Cr) for video compression; a video compression/reconstruction unit 5 for performing compression or reconstruction of the video signals incoming through the CCD 1 or transmission lines; a communication control and interface unit 6 for transmitting the compressed video signals or receiving video signals transmitted from another image communication system of the other party; and a controller 7 for controlling the overall circuit for the video signal processing, compression/reconstruction, and transmission.
The following description concerns the operation of the image communication system having such configuration.
Primarily, an image received through the lens 1a is converted into an electronic video signal in the CCD 1 and then input into the CCD driver 2.
The CCD driver 2 then performs analog signal compensation with respect to the input video signal to provide a signal of a predetermined level to the digital signal processor 3.
Subsequently, the digital signal processor 3 performs clamp, RGB color control, and brightness control with respect to the input video signal to display the photographed image through the monitor 4.
At this time, the digital signal processor 3 transforms a luminance signal Y and a color signal Cb and Cr into a specified format for compression of transmitting image and provides the transformed signal to the video compression/reconstruction unit 5.
Thereafter, the video compression/reconstruction unit 5 compresses the video signal received from the digital signal processor 3 according to JPEG (Joint Photographic Coding Experts Group) or MPEG (Moving Picture Experts Group). The compressed video signal is transmitted via the communication control and interface unit 6 over a communication line to another image communication system of the other party.
Alternatively, a video signal received via the communication control and interface unit 6 from the image communication system of the other party is respectively reconstructed and signal processed in the video compression/reconstruction unit 5 and the digital signal processor 3 and then displayed through the monitor 4.
Such signal processing is controlled by the controller 7.
This image communication system basically has a function of controlling a compression ratio of the photographed video signal to comport with a video data transmission rate which means the number of image frames per unit time (frames/second) required for the video transmission.
Techniques for video compression and transmission have usually been standardized. JPEG for still picture compression and MPEG for moving picture compression are typical image compression methods. These compression techniques use a quantization table.
However, in case of compressing an image using the quantization table, the file size of compressed video data becomes larger as the image gets more complicated, so the video data transmission is time-delayed and the number of image frames which can be transmitted for the unit time decreases.
Consequently, the quantization table should be controlled to increase the compression ratio and decrease the file size before the transmission for the purpose of satisfying the required number of image frames per unit time.
However, when such compressed file is reconstructed in the other party""s system, an image of much degraded picture quality is displayed due to damage on the contour of the image (that is, an edge is not displayed in good order), block effect, or loss of reconstructed data which may be caused by over compression of the source image.
Briefly, the amount of data of a video signal to be compressed increases as the complexity of transmitting image gets larger. If the compression ratio is increased to deal with the large amount of data, the resultant compressed video signal is reconstructed to an image of degraded picture quality.
Particularly, in case of employing the NTSC (National Television System Committee) mode, several frames to 30 frames should be transmitted for a second. In this occasion, increase of compression ratio for a complex image may lead to a result quite different from a source image.
Besides, the conventional techniques perform contour compensation emphasizing edge components (high frequency components) of an image before compressing the image, so the contour compensated image is compressed, decreasing compression efficiency.
Accordingly, the present invention is directed to a video coding and decoding method for an image communication system that substantially obviates one or more of the limitations and disadvantages of the related art.
An objective of the present invention is to provide a video coding method for setting a reference compression ratio available for minimizing picture degradation, comparing a compression ratio of a present compressed image with the set reference compression ratio, and decreasing a complexity of a source image to control the present compression ratio to be smaller than the reference compression ratio if the present compression ratio exceeds the reference compression ratio in an image communication system, thereby minimizing degradation of picture quality when the image is received and reconstructed by another system.
Another objective of the present invention is to provide a video coding method for setting a video data transmission rate of a transmission line and a reference compression ratio for minimizing picture degradation, compressing a present image according to the transmission rate, comparing a present compression ratio of the compressed image with the reference compression ratio, and decreasing a complexity of a source image obtained before the compression to control the present compression ratio to be smaller than the reference compression ratio if the present compression ratio of the compressed image exceeds the reference compression ratio in an image communication system, thereby minimizing degradation of picture quality when the image is received and reconstructed by another system.
Still another objective of the present invention is to provide a video coding and decoding method for controlling video compensation values of a luminance signal and/or color signal to lower a complexity of a source image and transmitting an overhead containing data of controlled values together with the image so as to allow a system receiving and reconstructing the image to perform signal processing corresponding to the video compensation performed by a transmitting party, thereby minimizing degradation of picture quality of the transmitting image and increasing transmission efficiency.
Additional features and advantages of the invention will be set forth in the following description, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure as illustrated in the written description and claims hereof, as well as the appended drawings.
To achieve these and other advantages, and in accordance with the purpose of the present invention as embodied and broadly described, a video coding method for image communication, comprises: the communication limit condition setting step of setting a specified reference compression ratio for minimizing degradation of picture quality which may occur during reconstruction; the video compressing step of compressing a present image changing a compression ratio according to a transmission rate; the step of comparing a present compression ratio of the compressed image with the set reference compression ratio; and the step of performing compensation value control with respect to an image obtained before the compression to lower a complexity of the image if the present compression ratio of the compressed image is larger than the reference compression ratio and performing the above compression with respect to the compensation value controlled image.
Overhead information indicating a content on compensation of the video signal is generated and transmitted with the compressed video signal.
The overhead information indicates whether or not contour compensation of the image was performed and which of emphasizing and attenuating compensations was performed if the contour compensation was performed.
The video signal compensation value control is performed such that at least one of a horizontal/vertical contour compensation value, a low luminance compensation value, a processing reference value of low data in case of data coring compensation and the amount of attenuation of a color value in case of chroma suppress compensation is controlled.
In another aspect, the present invention provides a video decoding method of an image communication system, comprising: the overhead reading step of detecting from a received data bitstream, information indicating whether or not contour compensation was performed and which of emphasizing and attenuating compensations was performed; the contour compensation step of performing the contour compensation with respect to the received video data if it is determined that the received video data was not contour-compensated or the attenuating compensation was performed with respect to the received video data according to a result of the overhead reading step; and the step of reconstructing the received video data as it is without performing the compensation if it is determined that the contour emphasizing compensation was performed with respect to the received data according to the result of the overhead reading.
The present invention previously sets a compression ratio as a communication limit condition to minimize degradation of picture quality that may occur during the reconstruction.
At this time, a maximum data size of one frame in accordance with the video transmission rate (frame/second) of a transmission line is also set.
In the conventional art, a source image is so compressed as to comport with the video transmission rate, namely, the maximum data size of one frame that is set in accordance with the video transmission rate. If a size of the compressed video data exceeds the maximum data size, a quantization table value is controlled to increase the compression ratio, thus satisfying the condition of transmission rate. Finally, the compressed image is transmitted. However, if the source image is over compressed at the increased compression ratio, the picture quality is degraded when the image is reconstructed. The present invention decreases a complexity of the source image instead of increasing the compression ratio to minimize the degradation of picture quality.
In other words, a compression ratio available for minimizing the picture quality degradation is set as a reference compression ratio for the communication limit condition.
Thereafter, the source image is compressed in accordance with the transmission rate. If the compression ratio of the compressed image exceeds the reference compression ratio, it can be estimated that the degradation of picture quality will occur. In this occasion, luminance and color signals of the source image are properly compensated to decrease the complexity of the source image. The image whose complexity was decreased is compressed in accordance with the transmission rate.
The image whose complexity was decreased is a little different from the source image, but the operation for decreasing the complexity of the source image does not influence the entire picture quality. Therefore, when the video signal whose complexity was decreased is compressed, the compression ratio falls within the range smaller than the reference compression ratio. Since the video signal is compressed at the available compression ratio considering the degradation of picture quality, the degradation of picture quality caused by over compression can be prevented when this video signal is received and reconstructed.
The operation of lowering the complexity of the source image is usually achieved by proper control of a compensation value in a luminance signal processing step or a color signal processing step carried out at a digital signal processor.
Specifically, in the luminance signal processing step, the compensation of horizontal or vertical contour of a source image is achieved by controlling contour compensation data, thereby lowering the complexity of the source image.
In the luminance signal processing step, the compensation of low luminance signal of the source image is achieved by controlling a corresponding compensation value, thereby lowering the complexity of the source image.
In the luminance signal processing step, data coring is achieved by controlling a coring reference value, thereby lowering the complexity of the source image.
Additionally, in the color signal processing step, chroma suppress is achieved by controlling the size of color data, thereby lowering the complexity of the source image.
Meanwhile, during the video compensation for lowering the complexity of the source image in coding the image, information indicating whether or not the contour compensation is carried out or whether or not attenuating compensation is carried out is generated as an overhead and transmitted with a video data bitstream. The receiving party detects the overhead and performs an operation corresponding to the process performed by the transmitting party.
Typically, the contour of an image contains a lot of edge components. To compress the edge components (high frequency components), a high compression ratio is required. This causes the degradation of picture quality and the decrease of transmission efficiency. Therefore, the transmitting party compresses and transmits the source image without performing the contour compensation and then the receiving party performs the contour compensation when reconstructing the source image, using the overhead indicating the fact that the contour compensation was not performed in the transmitting party.
Alternatively, if the transmitting party performs the contour compensation, information indicating that the compensation is emphasizing compensation or attenuating compensation is generated as the overhead and transmitted to the receiving party. The receiving party then reconstructs the source image as it is if it is determined that the contour compensation is the emphasizing compensation and performs the emphasizing compensation if it is determined that the contour compensation is the attenuating compensation, thereby minimizing the degradation of picture quality of the source image and increasing data transmission efficiency.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.